1. Field of the Invention
The present invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate.
2. Description of Related Art
The term xe2x80x9cpatterning meansxe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
One of the most challenging requirements for micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of tables. For example, sub-100 nm lithography demands substrate- and mask-positioning stages with dynamic accuracy and matching between machines to the order of 1 nm in all 6 degrees of freedom (DOF), at velocities of up to 2 msxe2x88x921.
A popular approach to such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table) with micrometer accuracies but travelling over the entire working range, onto which is cascaded a fine positioning module. The latter is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers, but only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in all 6 DOF, large-range motions are rarely required for more than 2 DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy required for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schxc3xa4ffel et al xe2x80x9cIntegrated electro-dynamic multi-coordinate drivesxe2x80x9d, Proc. ASPE Annual Meeting, California, USA, 1996, p.456-461. Similar encoders are also available commercially, e.g. position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH. Although such sensors can provide sub-micrometer level resolution without difficulty, absolute accuracy and in particular thermal stability over long travel ranges are not readily achievable.
Position measurement for the mask and substrate tables at the end of the fine positioning module, on the other hand, has to be performed in all 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability over the entire working range. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
Although the technology behind such interferometer systems is very mature, their application is not without problems. One of the most significant drawbacks of the interferometer is the dependence of wavelength on environmental pressure and temperature, as described by Schellekens P. H. J. xe2x80x9cAbsolute measurement accuracy of technical laser interferometersxe2x80x9d Ph.D. Thesis, TU Eindhoven, 1986, which is given by:                               λ          a                =                              λ            v                    η                                    (        1        )            
where:                                                                                           (                                      η                    -                    1                                    )                                                  P                  ,                  T                  ,                  H                  ,                  C                                            =                                                                    D                    xc3x97                    0.104126                    xc3x97                                                                  10                                                  -                          4                                                                    ·                      P                                                                            1                    +                                          0.3671                      xc3x97                                                                        10                                                      -                            2                                                                          ·                        T                                                                                            -                                  0.42066                  ⁢                                      xe2x80x83                                    xc3x97                                                            10                                              -                        9                                                              ·                    H                                                                                                                          D              =                              0.27651754                ⁢                                  xe2x80x83                                xc3x97                                  10                                      -                    3                                                  xc3x97                                  [                                      1                    +                                          53.5                      xc3x97                                              10                                                  -                          8                                                                    ⁢                                              (                                                  C                          -                          300                                                )                                                                              ]                                                                                        (        2        )            
P: atmospheric pressure [Pa]
T: atmospheric temperature [xc2x0 C.]
H water vapor pressure [Pa]
C CO2 content [ppm]
This remains one of the major problems in the thermal design of an optical lithography system. Typically, both temperature and pressure along the optical path of the interferometer has to be actively controlled to mK and mbar levels by the use of dry, clean (to better than Class 1) air, e.g. supplied by air showers.
In addition, the mounting adjustment of multi-axis interferometers for orthogonality and coplanarity, as well as the subsequent calibration procedure to remove any residual errors, are both extremely complex and time consuming. Even after such adjustments and calibration procedures, the measurement is only accurate if the relative positions of the interferometer blocks remain stable. The nanometer dimensional stability requirements of the metrology frame, on which the interferometer blocks are mounted, imply that the metrology frame has either to be made out of a material with low or zero coefficient of thermal expansion (CTE), such as Invar or Zerodur, or active thermal stabilization to mK levels, or both. Furthermore, the pointing stability of the laser beam during operation may introduce additional cosine or Abbe errors which need to be calibrated out on a regular basis by some form of automated routine.
An interferometer system is of course only a relative measuring system, capable of measuring changes in length (of optical path, to be precise). A zero reference in each degree of freedom can only be generated with additional equipment, such as so-called alignment sensors as described in WO 98/39689.
Although metrology frames in state-of-the-art lithography systems are highly isolated from ambient vibration, thermal deformation of the order of 0.5xc3x9710xe2x88x929 m is not totally avoidable. It is, therefore, desirable that the position of the substrate or mask tables be measured directly relative to the optical imaging system. Mounting of interferometers directly on the lens, for example, is both difficult and undesirable. Relative length measurement to the lens can, however, still be realized by differential interferometry, at the expense of the added complication and cost.
The multiple beams required for such 6 DOF interferometric measurement cannot be adequately supplied with sufficient optical power by one laser source, thus requiring multiple sources with additional wavelength matching demands. The total thermal dissipation of the lasers and detectors combined exceeds 50W, which is well above the level allowable for the dimensional stability of the metrology frame. Both the lasers and the detectors have thus to be mounted remotely via optical links.
As can be seen, whilst the resulting interferometry based system is technically viable and has been implemented in practice, it is by no means simple, robust and economical.
The most obvious alternative to interferometers for long-range displacement measurements with micrometer or nanometer resolutions is the optical incremental encoder. Optical encoders with sub-nanometer resolutions have become available in recent years and have been promoted as viable alternatives to single-axis interferometry. The sub-nanometer resolution is achieved by using fine-pitched gratings (down to 512 nm) in combination with interpolation techniques (up to 4096xc3x97). Most of such encoders, however, provide length measurement in 1 DOF only. As such, they do not lend themselves readily to nano-metrology in all 6 DOF simultaneously. Amongst the difficulties is the high level of crosstalk of the displacement signal to parasitic movements in the other 5 DOF.
It is an object of the invention to provide an improved displacement measuring system for use in a lithographic projection apparatus, and especially a system in which problems suffered by existing systems are solved or ameliorated.
According to the invention there is provided a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate; and
a displacement measuring system for measuring the position of a moveable object comprising one of said support structure and said substrate table in at least two degrees of freedom, said displacement measuring system comprising at least one grid grating mounted on said moveable object and at least one sensor head for measuring displacements of said grid grating in two degrees of freedom.
The invention also provides a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate; and
a displacement measuring system for measuring the position of a moveable object comprising one of said support structure and said substrate table in at least two degrees of freedom, said displacement measuring system comprising at least one grid grating mounted on a reference frame and at least one sensor head mounted on said moveable object for measuring displacement of said moveable object relative to said grid grating in two degrees of freedom.
A major advantage of the 2D grid encoder is that the measurement grid can be permanently fixed on a grating plate. Even if the grating is not perfectly orthogonal, straight or linear, this remains unchanged as long as the grating plate is free from distortions (either thermal or elastic). Such linearity or orthogonality errors can be calibrated out without too much difficulty by, for example, vacuum interferometry. The calibration only needs to be performed once for each grating, or not at all if one is only interested in positional repeatability. The use of a grid encoder essentially removes the guideway straightness and orthogonality from the error budget, when compared with single-axis encoder-based solutions.
The present invention can therefore provide an alternative solution to interferometry, at least in 3 coplanar degrees of freedom (X, Y, Rz), by combining the principles of grid gratings and sub-nanometer encoding.
To address the issue of output sensitivity to parasitic movements in the remaining degrees of freedom of encoders with nanometer resolutions, systems used in the present invention make use of the interference pattern of the first order diffraction of the collimated incidence light from a monochromatic source off the grating. This method ensures that the signals at the detector are free from high-order harmonics, making it possible to perform very high interpolation without incurring excessive errors. In addition, it allows a much larger position latitude of the reading head relative to the grating in the non-measurement directions. For more information on such a detector see U.S. Pat. No. 5,643,730, which document is hereby incorporated herein by reference.
A typical system used in the present invention comprises a grid grating with a period of 10 xcexcm or less, with an interferential reading (encoder) head in 2 DOF and an interpolator of up to a factor of 20,000 for each axis.
For the measurement of the remaining 3 DOF, namely Z, Rx and Ry, various short range displacement sensing technologies can be employed, including optical triangulation, fiber-optic back-scatter, interferometric sensors (which can have a very short optical path in air and therefore be much less sensitive to environmental fluctuations), capacitive or inductive sensors.
Currently, capacitive and optical sensors are preferred to the other measuring principles, though the others may be appropriate in some applications of the invention. The use of inductive sensors against a Zerodur chuck is problematic, as conductive targets are required for the sensors. Pneumatic proximity sensors (air micrometer), on the other hand, suffer from limited resolution and working distance, as well as exerting a finite force on the target.
Optical sensors, whether interferometric or triangulated, can be designed with a relatively large (a few millimeters) working distance, which helps to ease assembly tolerances. Compared to capacitive sensors, they usually have higher bandwidths, and can be configured as an absolute distance sensor. As an absolute sensor, however, they do suffer from long-term stability problems due to mechanical drifts (thermal or otherwise) requiring periodic calibration.
Capacitive sensors, on the other hand, can be designed as an absolute sensor with very high stability. Furthermore, the distance measurement is performed over a relatively large target surface, which helps to reduce any effects of localized unevenness of the target surface. Despite their limited measurement range and stand-off clearance, they are currently the preferred choice in lithographic applications.
An encoder based nano-positioning system offers an advantageous alternative to interferometry and is much simpler to implement. Better measurement stability can be achieved by the fact that the measurement grid in the X-Y plane is permanently fixed onto the mask table, which when implemented in a zero-CTE material, such as Zerodur, is both long-term dimensionally stable and thermally insensitive. This eases considerably the stringent demand on environmental control of the area immediately around the optical path of the interferometer beams, particularly in the case of a lithographic projection apparatus employing wavelengths of 157 nm or below. Such devices require to be purged with gas, that does not absorb the beam (which is strongly absorbed in air), and by avoiding the need for air showers over the length of the interferometer beams, the present invention can substantially reduce consumption of purge gas.
The mask position relative to the projection optics can also be measured in the encoder solution without resorting to a differential configuration. Although placing the reading head directly on the top of the projection optics does put more demands on the thermal dissipation of the former, techniques to minimize this such as active cooling or remote light source and detectors linked by optical fibers are already available and already deployed in state-of-the-art interferometer systems.
The invention also provides a device manufacturing method which comprises:
measuring displacements of one of a support structure and a substrate table in at least two degrees of freedom using at least one grid grating mounted thereon and at least one sensor head.
The invention further provides a method of calibrating a lithographic projection apparatus comprising the steps of:
providing a reference pattern to patterning structure held in a moveable support structure, said reference pattern having a plurality of reference marks at pre-calibrated positions in at least a scanning direction of the lithographic projection apparatus;
holding an image sensor on a substrate table at a constant position relative to the projection lens;
positioning said support structure so as to project an image of each of said reference marks in turn onto said transmission image sensor; and
measuring the position of said support structure in at least a first degree of freedom when each of the reference marks is projected onto said image sensor.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9cexposure areaxe2x80x9d or xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The invention is described below with reference to a coordinate system based on orthogonal X, Y and Z directions with rotation about an axis parallel to the I direction denoted Ri. The Z direction may be referred to as xe2x80x9cverticalxe2x80x9d and the X and Y directions as xe2x80x9chorizontalxe2x80x9d. However, unless the context otherwise demands, this should not be taken as requiring a specific orientation of the apparatus.